Amplifying arrangement for voltages, particularly for automatic volume control



May 28, 1946. c. T. SCULLY 2,400,956

AMPLIFYING ARRANGEMENTS FOR VOLTAGES, PARTICULARLY FOR AUTOMATIC VOLUMECONTROL Filed Feb. 16, 1945 Patented May 28, 1946 AMPLIFYING ARRANGEMENTFOR VOLT- AGES, PARTICULARLY FOR AUTOMATIC VOLUME CONTROL Charles ThomasScully, London W. C. 2, England,

assignor, by mesne assignments, to International Standard ElectricCorporation, New

York, N. Y., a corporation of Delaware Application February 16, 1943,Serial No. 476,112 In Great Britain February 20, 1942 4 Claims.

The present invention relates to electrical voltage amplifyingarrangements, and more particularly applies to automatic volum controlsystems.

It is well known that in electrical communication systems, it is Veryoften necessary to provide means for dealing with variations in thesignal amplitude, caused for example, by variations in the transmissioncharacteristics of the communication medium due to weather changes andthe like. Such means frequently comprise automatic arrangements foradjusting the gain of one or more amplifiers in the system in accordancewith the changes in the signal level, so that the level at the outputterminals of the amplifier remains substantially constant. Theseautomatic volume control arrangements may be operated by the signalsthemselves, or by a'separate pilot signal provided for the purpose, andsubjected to the same changes in the communication medium.

Automatic volume control arrangements are frequently used in radioreceivers to counteract fading, and the present invention is ofparticular (though not exclusive) application to such apparatus.

' It is generally the practice to obtain from the incoming signal (orpilot signal) a rectified voltage proportional to the signal amplitude,and to apply this rectified voltage to control the amplification 'ofsome convenient valve in one of the amplifiers of the system, forexample by means of a control grid. The control voltage is obtained bytapping off the signals, possibly at an intermediate frequenc stage,then amplifying, and then rectifying the amplified signals. It isusually necessary also to amplify the rectified signals, and for thispurpose a valve is required whose high tension supply must be derivedfrom a separate direct current source.

The present invention provides a simpler means for obtaining a largecontrolling voltage or power by avoiding the necessity for any largeamount of high frequenc amplification or for a separate direct currentsource.

The invention accordingly comprises an arrangement for obtainingacontinuous volume control voltage for signals transmitted over acommunication medium of variable transmission characteristics, in whicha rectifier is provided for obtaining a continuous voltage from analter- .nating current supply source to serve as the said controlvoltage, the output of the said rectifier being controlled by arectified voltage obtained from the said signals, or from a separatepilot signal transmitted over the same medium.

The invention will be more clearly understood from the followingdetailed description which refers to the accompanying drawing Figs. 1and 2 of which show schematic circuit diagrams of two embodiments.

Referring to Fig. l, the signals for operating the automatic volumecontrol are tapped off the channel through the transformer 2 connectedto the anode and cathode of a di de I through a resistance 3 shunted bya condenser 4. A rectified voltage proportional to the amplitude of thesignals will be developed across resistance 3 the alternatingcurrentsbeing by-passed by the condenser 4; and the cathode end of theresistance will be positive to the other end. The resistance 3 isconnected between the cathode and control grid of a valve 5, a gridbiasing'battery I2 being included as shown, having suitable polarity andvoltage to adjust the position of the operating point on the valvecharacteristic. Any other convenient biasing arrangement may be used.The cathode of valve 5 is connected to earth, and the anode is connectedthrough an appropriate load resistance 6 to one terminal of a suitablealternating current supply (such as a transformer connected to thecommercial supply) the other terminal of which is earthed, The anode isalso connected to an output terminal I0 through a smoothing networkcomprising the series resistance l, and the shunt condensers 8 and 9.The second output terminal H i connected to earth, and the voltagedifference between I0 and II is applied in any convenient known way tocontrol the amplifier gain.

It will be seen that the valve 5 acts a rectifier to produce acontinuous voltage from the alternating current supply, the magnitude ofwhich is controlled by voltage developed by the incoming signal acrossresistance 3. Owin to the load re: sistance 6, and on account of thefact that there will be substantiall no anode current in the valve 5 forthe negative half-waves, the anode voltage will be practically the sameas the applied voltage for the negative half-waves. Owing, however, tothe current which flow during the positive half-waves, the anode voltagewill be lower than the applied voltage. Thus in the rectified output,terminal l0, which is connected to the anode, will be negative toterminal ll.

When the signal amplitude increases, the voltage across the resistance 3increases, making the control grid of valve 5 more positive. This willreduce the anode voltage for the positive halfwaves, and the negativevoltage of terminal III will be accordingly increased, and the changewill be amplified by the valve 5. n the basis that an increase in thenegative voltage is a negative change, the arrangement may be consideredto roduce a change of the controlling voltage in the opposite sense tothe change in the signal level, or alternatively it may be consideredthat a phase change of 180 degrees is produced. If this phase change isinconvenient, the diode I may be reversed so that the anode instead ofthe cathode is connected to the control grid of the valve 5. An increasein the signal amplitude will then cause a reduction in the negativecontrol voltage.

Fig. 2 shows a modification of Fig. 1 in which the valve 5 has beeninverted in order to produce a positive instead of a negative controlvoltage on terminal Ill. The anode is connected to earth and the cathodeto the load resistance 6, the resistance 3 being connected between thecathode and the control grid. The rectified voltage of the cathode willnow be positive to the anode,

and terminal 10 will now be positive to terminal ll.

When the diode is connected as shown in Fig. 2, an increase in thesignal level makes the control grid more negative. This reduces thenegative rectified voltage so that the positive voltage of terminal illwith respect to terminal II is also reduced, and a phase change of 180degrees (that is, a change in the opposite sense) will be produced as inthe case of Fig. 1. By reversing the diode an increase of the signallevel will be made to increase the positive voltage of terminal l0, andthe change will be in the same sense.

It will thus be seen that the connection of the valve 5 decides thepolarity of the control voltage, and the connection of the diode Idenotes whether or not a phase change will be introduced, so that anarrangement suitable for any particular case.

can be produced.

Various minor modifications of the arrange: ment shown in theaccompanying figures are possible to suit practical cases. For example,the diode I could be replaced by a triode or pentode suitably biassedfor rectification, or a metal rectifier could be used. The smoothingnetwork I, 8,

means for this purpose.

9, could be replaced by a low pass filter having the cutoif frequency alittle below the frequency of the supply. It might also be necessary toconnect a low pass filter between the resistance 3 and the control gridof the valve 5 for excluding the signal or pilot frequency. Further, thegrid biassing arrangement, shown as a battery l2 in the figure, could beone of any of the well known It is not intended that the inventionshould be limited to the embodiments shown and described.

What is claimed is:

1. System for obtaining a continuous volume control voltage for signalstransmitted over a communication medium of variable transmissioncharacteristics, including a rectifier fed by said signals, a thermionicvalve, means connecting the output of said rectifier to the inputelectrodes of said valve, a source of alternating current supply, oneterminal of said alternating current supply being connected to thecathode of said valve, and the other terminal to the anode thereof, aload resistor in series with said supply, and an output circuitconnected directly between said cathode and said anode so as to shuntsaid electrodes.

2. System according to claim 1, including a resistance in the outputcircuit of said rectifier, said thermionic valve having its control gridand cathode connected respectively to the terminals of said resistance.

3. System according to claim 1, including a resistance in the outputcircuit of said rectifier, in which the control grid of said thermionicvalve is connected to the junction point of said resistance and saidrectifier, whereby the rectifier connections are so poled that thechanges in the signal amplitude cause the control voltage to change inthe opposite sense.

4. System according to claim 1, including a resistance in the outputcircuit of said rectifier, in which the cathode of said thermionic valveis connected to the junction point of said resistance and saidrectifier, whereby the rectifier connections are so poled that thechanges in the signal amplitude cause the control voltage to change inthe same sense.

CHARLES THOMAS SCULLY.

